It is well known from various connections to utilize automatic transmissions to adapt the power and the speed from a motor to an outgoing propeller shaft, so that the motor, which may have a rather narrow range of rotational speed at which it operates really well, shall be able to function efficiently under varying conditions of operation.
Such transmissions are used for motor powered vehicles, like small and large cars, hereunder trucks, in large and small boats, for two-wheeled vehicles and in a lot of connections industrially where power transmissions arc needed. Such transmissions are especially important in combination with engines that work well within only a narrow range of rotational speed, which is typically for petrol engines.